1. Field of the Invention
The present invention relates to a polishing head and a polishing apparatus to polish the surface of an object to be processed such as a wafer.
2. Description of the Related Art
For example, wafer manufacturing steps include a polishing step of mirror-finishing the surface of a wafer. In this step, a wafer polishing apparatus brings a wafer into press contact with the surface of a rotating polishing pad to polish the surface of the wafer.
This wafer polishing apparatus has a polishing table rotated by a drive shaft. A polishing pad is arranged on the upper surface of the polishing table, and a polishing head which rotates while holding a wafer is arranged at a position opposing the polishing surface of the polishing pad.
FIG. 10 is a sectional view of a conventional polishing head. Reference numeral 105 in FIG. 10 denotes the polishing pad.
As shown in FIG. 10, the polishing head has a head body 100. A compression chamber 101 is formed in the head body 100, and a lower-surface opening 102 of the compression chamber 101 is sealed with a rubber film 103. A wafer U is held on the lower surface of the rubber film 103. The wafer U is surrounded by a ring-like retainer 104 fixed on the lower end face of the head body 100. The retainer 104 extends to the inside of the head body 100 in the radial direction. The outer peripheral portion of the rubber film 103 is supported on the upper surface of the retainer 104.
When the wafer polishing apparatus having the above configuration is to be used, the rotating polishing head is moved downward to bring the retainer 104 held on the lower end face of the head body 100 into press contact with the surface of the polishing pad 105. At this time, when a gas is supplied into the compression chamber 101 to expand the rubber film 103, the wafer U adhesively fixed to the lower surface of the rubber film 103 is brought into press contact with the rotating polishing pad 105, so that the surface of the wafer U is polished.
In the wafer polishing apparatus having the configuration, the outer peripheral portion of the rubber film 103 is held on the upper surface of the retainer 104. For this reason, depending on the vertical thickness of the retainer 104, the entire surface of the wafer U cannot be polished at a uniform polishing rate.
More specifically, as shown in FIG. 11A, when the thickness of the retainer 104 is large, the level at which the rubber film 103 is supported is higher than the upper surface of the wafer U, and a desired force cannot be loaded on the edge portion of the wafer U. For this reason, a necessary contact pressure cannot be given between the edge portion of the wafer U and the polishing pad 105, so that the edge portion of the wafer U is less polished than the central portion of the wafer U.
As shown in FIG. 11C, when the vertical thickness of the retainer 104, a level at which the rubber film 103 is supported is lower than the upper surface of the wafer U, and the edge portion of the wafer U is overloaded. For this reason, a contact pressure between the edge portion of the wafer U and the polishing pad 105 excessively increases to polish the edge portion of the wafer U more than the central portion. That is, circumferential droop disadvantageously occurs.
Therefore, in order to polish the entire surface of the wafer U at a uniform polishing rate, as shown in FIG. 11B, the vertical thickness of the retainer 104 must be appropriately set.
However, since the retainer 104 is always in slidable contact with the polishing pad 105 when the wafer U is polished, the vertical thickness of the retainer 104 gradually decreases by abrasion. For this reason, even though the vertical thickness of the retainer 104 is appropriately set depending on the thickness of the wafer U in an initial state, the position at which the rubber film 103 is supported comes down to cause circumferential droop.
In this manner, in order to polish the entire surface of the wafer U at a uniform polishing rate, the retainer 104 must be selected depending on the thickness of the wafer U. Furthermore, an amount of abrasion of the retainer 104 must be always monitored. For this reason, the wafer polishing apparatus having the configuration disadvantageously increases the load on an operator.
In recent years, as an apparatus which can polish the entire surface of the wafer U at a uniform polishing rate even though the retainer is worn, a separation type polishing head which holds the wafer U to make it possible to move the wafer U up and down based on the retainer is disclosed (for example, see Patent Application National Publication (Laid-Open) No. 2002-527893).
FIG. 12 is a sectional view of a conventional separation type polishing head.
As shown in FIG. 12, the polishing head has a head body 201 which is rotatably driven. The head body 201 has a recessed portion 201a formed in the lower surface of the head body 201. A ring-like retainer 202 is fixed to the outer peripheral portion of a portion of the head body 201 which is in contact with the polishing pad.
A plate-like support plate 203 is almost horizontally arranged inside the recessed portion of the head body 201. The plate-like support plate 203 is supported such that the plate-like support plate 203 can move up and down inside the head body 201. On the outer peripheral portion of the upper surface of the plate-like support plate 203, an isolation film 204 is arranged such that the isolation film 204 overlaps the outside of the outer peripheral portion in the radial direction. The isolation film 204 has flexibility. The edge portion of the isolation film 204 is supported by the head body 201.
In this manner, a first space 205 surrounded by the head body 201, the plate-like support plate 203, and the isolation film 204 is formed in the plate-like support plate 203 on the upper surface side. A first gas supply pipe 206 is connected to the first space 205. A gas is supplied from the first gas supply pipe 206 to the first space 205 to make it possible to pressure the upper surface of the plate-like support plate 203.
In addition, a recessed portion 207 is formed on the lower surface of the plate-like support plate 203. The recessed portion 207 is sealed with a rubber film 208. A second space 209 is formed between the plate-like support plate 203 and the rubber film 208. The wafer U is held on the lower surface of the rubber film 208. A second gas supply pipe 210 is connected to the second space 209. A gas is supplied from the second gas supply pipe 210 into the second space 209 to make it possible to pressure the lower surface of the plate-like support plate 203.
When the wafer U is polished by using the polishing head having the above configuration, the rotating head body 201 is moved down to bring the retainer 202 fixed on the lower end face of the head body 201 into press contact with the surface of a polishing head 211. A gas is supplied into the first space 205 and the second space 209 to adjust the pressures in the first and second spaces 205 and 209, so that the wafer U adhesively fixed on the lower surface of the rubber film 208 is brought into press contact with the polishing head 211.
In this manner, in the separation type polishing head, the head body 201 and the plate-like support plate 203 are independently driven. For this reason, even though the retainer 202 is worn to decrease the vertical thickness of the retainer 202, the level at which the rubber film 208 is supported is not adversely affected. As a result, the edge portion of the wafer U is not excessively polished, or, contrarily, the edge portion is not slightly polished. [Patent Application National Publication No. 2002-527893].
However, in polishing of the wafer U by using the polishing head, when the first space 205 or the second space 209 changes in pressure, the level of the plate-like support plate 203 changes, and the wafer U may not be preferably polished.
For example, when the pressure in the first space 205 is lower than that in the second space 209, the plate-like support plate 203 moves upward by a balance between pressures acting on the upper and lower surfaces of the plate-like support plate 203. At this time, since the rubber film 208 which holds the wafer U expands upward by the pressure in the second space 209 to trace an arc, a contact pressure acting between the edge portion of the wafer U and the polishing pad is lower than that of the central portion of the wafer U. As a result, the edge portion of the wafer U is not easily polished.
When the pressure in the first space 205 is higher than that in the second space 209, the plate-like support plate 203 moves downward due to the balance between pressures acting on the upper and lower surfaces of the plate-like support plate 203. At this time, the rubber film 208 which holds the wafer U shrinks upward to some extent by the pressure in the second space 209 to trace an arc. For this reason, a contact pressure acting between the central portion of the wafer U and the polishing pad is lower than that of the edge portion of the wafer U. As a result, the central portion of the wafer U is not easily polished.
FIG. 13 is a simulation graph showing a change in contact pressure in the radial direction of the wafer U when the pressure in the first space is changed. In this graph, a pressure given to the second space is fixed to 200 [hPa], and a pressure given to the first space is changed into [1] 205 [hPa], [2] 200 [hPa], and [3] 195 [hPa].
As shown in FIG. 13, when the pressure in the first space is [1] 205 [hPa], a contact pressure between the wafer U and the polishing pad at the central portion of the wafer U is about 200 [hPa]. In contrast to this the contact pressure sharply increases at the edge portion of the wafer U.
As also shown in FIG. 13, when the pressure in the first space is [3] 195 [hPa], a contact pressure between the wafer U and the polishing pad at the central portion of the wafer U is about 200 [hPa]. In contrast to this, the contact pressure sharply decreases at the edge portion of the wafer U.
In this manner, when the wafer U is polished by using a conventional polishing head, changes in pressure in first and second spaces cause a large difference between polishing rates of the central and edge portions of the wafer U.